Allow passing a venv to `uv pip --python` (#3064)
Fixes https://github.com/astral-sh/uv/issues/3060
## Summary
Allows passing a virtual environment (the path to the directory, rather
than the path to the Python interpreter within the directory) to the
`--python` option of the `uv pip` command.
## Test Plan
Tested manually to confirm that the expected new functionality works.
The test suite still passes after this change.
I don't know how to add tests for a new feature like this. I would be
happy to do so if someone can give me some pointers on how to do it.
